1. Field of the Invention
The present invention concerns a compound chamfering device that can chamfer the side or circumferential surface of a prism-shaped polycrystalline silicon ingot block or monocrystalline silicon ingot block of a raw material of a square or rectangular substrate to be used as the substrate of a solar cell (solar light-emitting electric panel). It also concerns a method for using this compound chamfering device to manufacture a smooth-surface ingot silicon ingot block by chamfering the four corners or the four side surfaces of an ingot block whose C-axis end faces are cut off. When the manufactured smooth-surface ingot silicon ingot block is sliced with a wire-cut saw to a thickness of 200-240 μm to simultaneously obtain many solar-cell silicon substrates, chipping or cracking will not occur in the resulting silicon substrate.
2. Description of the Related Art
In the process of manufacturing a solar-cell silicon substrate, four circumferential pieces of the circular cross-section of a cylindrical monocrystalline silicon ingot are cut away with a band saw to make a prism-shaped silicon ingot (workpiece) leaving an arc (R corner part) on the four corners, then it is supported in a clamping device consisting of the head stock and tail stock of a horizontal cylindrical grinder. The surface of the four sides is chamfered to the desired thickness (8-10 mm) with a cup wheel type grindstone, then it is sliced to manufacture square-shaped silicon substrates 200-330 μm thick (for example, see non-patent reference 1).
Also used as prism-shaped silicon ingot blocks are ingot blocks that are made by cutting a polycrystalline silicon ingot into 2 to 4 blocks. The polycrystalline silicon ingot is obtained by injecting a liquid of molten metal silicon into a prism-shaped graphite container, solidifying it unidirectionally, then chamfering with a band saw the lower end face and side that are contact-contaminated with the inner surface of the container. Prism-shaped monocrystalline silicon ingot blocks for solar cells are made, when semiconductor substrate production is slack, by cutting off with a slicer the four sides of a cylindrical silicon ingot block for semiconductor substrate manufacturing so as leave partially a round part, then chamfering both end faces, then doing corner round chamfering on the cylindrical ingot block (margin 7.5-8 mm), then chamfering the four side planes (margin 0.5-1 mm). A monocrystalline silicon substrate has a greater photoconversion efficiency than a polycrystalline silicon substrate, but is more difficult to chamfer.
For example, unexamined patent H8-73297 [1996] (patent reference 1) proposes a polycrystalline silicon ingot manufacturing method in which molten metal silicon, obtained by reducing quartz or quartz sand in an electric furnace, is poured into a heat-resistant column-shaped container, and slowly cooling it from the lower end of the container to the upper end in order to make a unidirectionally solidified prism-shaped polycrystalline silicon ingot rod. The lower end face and side that are contact-contaminated with the inner surface of the container are chamfered by grinding and polishing with a margin of 5 mm, then etching is done with an aqueous solution of a mixture of hydrofluoric acid and nitric acid.
U.S. Pat. No. 6,679,759 (patent reference 2) proposes a method in which the rough surface on the sides of a silicon ingot block, whose C-axis end face is cut off perpendicularly, is polished with a polishing tool to give it a surface smoothness Ry of 8 μm or less. The silicon ingot block is then made into a solar-cell silicon substrate 200-330 μm thick. This patent reference states that with a silicon block whose surface smoothness Ry is 8 μm or less, no cracking or chipping will occur in the substrate even if the silicon ingot block is simultaneously cut with a wire saw into solar-cell silicon substrates 200-330 μm thick.
In addition, unexamined patent 2009-99734 (patent reference 3) proposes a silicon wafer manufacturing method in which a silicon ingot formed by casting is cut into multiple silicon blocks and then the silicon blocks are sliced into many silicon wafers. The silicon wafer manufacturing method includes a grinding process in which, when the silicon ingot formed by casting is cut into multiple (2-4) silicon blocks, at least one surface of the silicon ingot is first ground flat, and a silicon block cutout process in which the silicon ingot is placed on a base with its surface that has been ground flat facing downward, and multiple silicon blocks are cut out from the silicon ingot.
Unexamined patent 2004-6997 (patent reference 4) proposes a method for manufacturing angular wafers in which a cylindrical silicon block for manufacturing silicon wafers for solar cells is chamfered with a band saw and made into a prism-shaped silicon block. The side planes are polished with a roll-type diamond sponge flat grindstone, and the block is then sliced.
Unexamined patent 2009-55039 (patent reference 5) proposes a method for manufacturing angular wafers in which a cylindrical silicon block is chamfered with a band saw to make a prism-shaped silicon block, the side planes are roughly polished with a cup wheel type grindstone whose abrasive grain diameter is 60-80 μm, and the side planes are finish polished with a cup wheel type grindstone whose abrasive grain diameter is 3-40 μm. The surface is further etched, and the block is then sliced.
The specification of U.S. Pat. No. 4,133,935 (patent reference 6) proposes a method for manufacturing roughly square-shaped thin silicon substrates in which a silicon ingot formed by casting undergoes cylindrical polishing to make its outer circumferential surface smooth, its four sides are cut away with a slicer or other side-peeloff machine to make a silicon ingot of roughly square cross section having four corners rounded off. The ingot is cut off to make multiple silicon ingot blocks, and in addition the four sides are polished flat with a polishing tool, making the smoothness Ry of the side 10-20 μm, and this silicon ingot block is cut off perpendicularly with a wire cut method.
Unexamined patent 2009-233794 (patent reference 7) proposes a method in which, when grinding/polishing the surface of a silicon block, the front and back of the silicon block in the longitudinal direction are held with a pair of chucking members (the head stock and the tail stock) that do chucking (clamping) mechanically. In this state, the sides of the silicon block and the four angles (the round corner parts on the four corners) that join them are ground and polished using a rough grinding grindstone and a precision-finishing grindstone. Because this method can keep the four angles and four sides of the silicon block in a state in which the chucking members are made to float in the air without making contact, any injury to the sides and angles can be prevented. Because the angles as well as the sides of the silicon block can be chamfered by grinding and polishing when manufacturing silicon wafers by slicing the silicon block, any nicking of the circumferential edge can be avoided and the yield can be improved.
It has been pointed out by substrate processing manufacturers that as the length of the side of a prism-shaped silicon ingot gets longer, from 50 mm to 125 mm, 156 mm, 200 mm, then 240 mm, when mass-producing solar-cell silicon substrates 200-330 μm thick by all at once slicing with a wire cut saw a prism-shaped silicon ingot of side 156 mm to 240 mm, as stated above, chipping sometimes occurs in the round corner part of the prism-shaped silicon ingot, thereby raising the silicon substrate production loss rate. The occurrence of chipping during cutting of the wafers is prevented by the treatment method of doing wire saw cutting after flat-polishing the sides with a polishing tool as in patent reference 3 and patent reference 6 above, by the method of polishing with a polishing brush as described in unexamined patent 2002-252188 (patent reference 8), or by the method of etching treatment.
Currently, it takes about 95-120 minutes to chamfer a prism-shaped monocrystalline silicon ingot of side 156 mm and height 250 mm whose four corners are cut off leaving rounded-off corner parts, and it takes about 180-210 minutes to chamfer a prism-shaped monocrystalline silicon ingot of side 156 mm and height 500 mm whose four corners are cut off leaving rounded-off corner parts. To this processing time, 10 minutes of transfer time are added to move the silicon ingot from the rough grinder to the finishing grinder.
In other patents, examined patent S49-16400 [1974] (patent reference 9), unexamined patent H4-322965 [1992] (patent reference 10), unexamined patent H6-166600 [1994] (patent reference 11), and unexamined patent H6-246630 [1994] (patent reference 12) propose a horizontal cylindrical grinder that chamfers the surface of a cylindrical silicon ingot for manufacturing silicon substrates for semiconductor substrates.
The horizontal cylindrical grinder disclosed in patent references 9 to 12 consists of a clamping mechanism consisting of a pair made up of a head stock that causes the center axis to rotate by a servomotor via a speed reduction mechanism and a tail stock that can move in the left-right direction; a raising-and-lowering mechanism that raises and lowers a grinding head axially supported on the grindstone shaft so that the shaft center of the cylindrical silicon ingot faces in a horizontal (side) direction by means of the head stock center and the tail stock center of this clamping mechanism, and the circular plane of the disk-shaped flat grindstone faces the upper surface of the circumference of the cylindrical ingot, which is rotatably supported; and a movement mechanism that causes the grinding head to move in a straight line left and right parallel to the shaft center of the cylindrical ingot.
In the cylindrical grinding of the cylindrical silicon ingot, the bottom of the disk-shaped flat grindstone is lowered by the raising-and-lowering mechanism to a height position for chamfering at the height position of the circumferential top of the rotating cylindrical ingot. By the linear movement mechanism, the grinding head is moved rightward. While causing the disk-shaped flat grindstone of the grinding head to rotate on the circumferential top of the cylindrical ingot, it is brought into contact with the cylindrical ingot and cutting-in begins. After the disk-shaped flat grindstone reaches the right-end position of the cylindrical ingot, the disk-shaped flat grindstone is lowered by the raising-and-lowering mechanism by the height of the cutting-in amount. The direction of movement of the disk-shaped flat grindstone is reversed to leftward by the linear movement mechanism. After the disk-shaped flat grindstone reaches the left end position of the cylindrical ingot, the disk-shaped flat grindstone is lowered by the raising-and-lowering mechanism by the height of the cutting-in amount. The grinding head is moved rightward by the linear movement mechanism and the disk-shaped flat grindstone reaches the right-end position of the cylindrical ingot. The disk-shaped flat grindstone is lowered by the raising-and-lowering mechanism by the height of the cutting-in amount. The direction of movement of the disk-shaped flat grindstone is reversed to leftward by the linear movement mechanism. After the disk-shaped flat grindstone reaches the left-end position of the cylindrical ingot, likewise below there is repeated lowering, reversal, chamfering, lowering, reversal, chamfering of the disk-shaped flat grindstone, and chamfering is done to the desired thickness (10 μm to 5 mm).
The patent applicant of this application has proposed, in the specification of patent application 2009-296602 (patent reference 13), a compound chamfering device that will make it possible to quickly manufacture a prism-shaped silicon ingot block with no occurrence of chipping during wire cutting, in which a workpiece loader is attached to a chamfering device having a workpiece loading/unloading stage, a workpiece side rough grinding stage, a workpiece side finishing grinding stage, and a workpiece four-corner rounding-off finishing grinding stage.
The patent applicant of this application has also proposed, in the specification of patent application 2010-61844 (patent reference 14), an ingot block compound chamfering device 1 (see FIG. 5 and FIG. 6) that is characterized in that it has
a) a workpiece table 4 provided so as to allow left-right back-and-forth movement on guide rails provided in the left-right direction on a machine frame 2,
b) a clamping mechanism consisting of a pair made up of a head stock 7a and a tail stock 7b that are mounted separately on the left and right on this workpiece table,
c) a drive mechanism 5 that causes left-right back-and-forth movement of the workpiece table 4 on which is mounted the clamping mechanism, which holds a workpiece (ingot block) w, being the direction in which one sees the workpiece table perpendicularly from the front side, and facing from the left-side direction to the right-side direction,d) an ingot block side-peeloff stage 90 on which are provided, in front of and in back of the workpiece table with the workpiece table in between, a pair of rotary blades (slicer blades) 91a, 91b axially supported on a pair of spindles 92a, 92b that can move forward and backward, so that their diameter planes face each other,e) a first grinding stage 11 in which a pair of cup wheel type grindstones 11g, 11g axially supported on a pair of grindstone shafts that can move forward and backward are provided in front of and behind the workpiece table with the workpiece table between them, in such a way that the grindstone planes face each other,f) a second grinding stage 10 that is provided parallel to the right horizontal side of the first grinding stage and in which a pair of cup wheel type grindstones 10g, 10g axially supported on a pair of grindstone shafts that can move forward and backward are provided in front of and behind the workpiece table with the workpiece table between them, in such a way that the grindstone planes face each other,g) a load port 8 that is on the right horizontal side of the second grinding stage and has, in the housing material positioned on the front side of the workpiece table, an opening through which the workpiece can be moved into and out of the clamping mechanism, and,h) on the rear side of the workpiece table that is opposite the load port 8, a round corner part finishing grinding stage 9 in which a grindstone shaft having a grindstone wheel is parallel to the left-right direction of the workpiece table, and this grindstone shaft is provided on a tool table in such a way that its shaft center can move forward and backward.